First of all, we must realize that this question assumes that the mainstream media is capable of causing us to simply believe what they say. This is not necessarily the case. There are many people who are capable of thinking for themselves. If we assume that people actually do simply believe what the media tells them, there are two main reasons for this.
First, there is a reason that is connected to the media. The media has, for years, presented itself as an objective source of news. Because of this, people have come to think that it actually is objective and unbiased. This means that they are likely to accept what the media says because they do not think that the media would be untrustworthy.
Second, and perhaps more important, is a reason that is connected to the people more than it is to the media. This is the fact that people typically do not want to work any harder than they have to especially when they feel confused or overwhelmed by an issue. It is very difficult to engage in your own research about any particular topic. If I wanted to find out something the NSA surveillance program that is not reported in the media, I would have a very hard time. I do not even know how I would start to do that. Believing what the media says is simply the path of least resistance and therefore that is what people do.
In short then, to the extent that people really do simply accept what the mainstream media says, it is partly because of the image the mainstream media has created for itself and partly because we are prone to doing things the easy way if at all possible.